I want to start experimenting with conductive thread for e-fashion. I was just wondering how hot it gets when voltage is applied? Presumably this is something I can control with the voltage chosen but what's normal for e-textile projects and any tips to keep it cool enough next to human skin?

As the resistance is very low, current will get quite high when a voltage is applied directly to the thread only without a certain load. A load could be a LED and a resistor in series, which increases the resistance and limits the current.

The power dissipated by the thread is in formula

Power = Voltage x Current

Every material has a specific heat (how much energy to add to raise 1 gram 1 degree) so depending on the length and mass of the thread the temperature will raise more or less. With a high current this can be alot so it is important to test your application before you weave it into some fabric as the fabric can be damaged or even catch fire. Double checking your electric design is seldom a luxury.

Advice: when you test an E-textile - first time or after modification - you or any other person should NOT wear it. Verify proper working and safety first.